Catalyst body

ABSTRACT

A catalyst body of the present invention includes: a porous carrier in which a large number of aggregate particles containing a main component of a nonoxide ceramic are bonded to one another while a large number of pores are disposed; and a catalyst layer carried on the porous carrier and containing a compound of an alkali metal, wherein the porous carrier has an oxide film unavoidably formed on a part of the surface of the aggregate particles, and an oxide film protective layer formed of a material which does not form low-melting glass with the alkali metal is further disposed between the oxide film and the catalyst layer in such a manner as to coat at least a part of the oxide film.

TECHNICAL FIELD

The present invention relates to a catalyst body preferably usable forpurification of an automobile exhaust gas, particularly to a catalystcapable of effectively preventing a drop of catalyst activity in a casewhere a nitrogen oxide trap catalyst such as an alkali metal is carriedon a catalyst carrier formed of a nonoxide ceramic or the like.

BACKGROUND ART

In recent years, automotive exhaust gas regulation has beenstrengthened, a lean-burn engine, a direct jet engine and the like havespread, and accordingly a nitrogen oxide trap catalyst (hereinafterreferred to as “NO_(x) trap catalyst”) has been put to practical use,which is capable of effectively purifying nitrogen oxide (NO_(x)) in anexhaust gas in a lean atmosphere. The NO_(x) trap catalyst containseffective components such as alkali metals (potassium (K), sodium (Na),lithium (Li), cesium (Cs), etc.), alkali earth metals (barium (Ba),calcium (Ca), etc.), and rare earths (lanthanum (La), yttrium (Y),etc.). Especially, barium has been broadly used from the beginning ofthe practical use of the NO_(x) trap catalyst. In recent years, additionof potassium has been attempted which is superior in a nitrogen oxidetrap ability (hereinafter referred to as “NO_(x) trap ability”).

This NO_(x) trap catalyst is usually used in the form of a catalystcarried on a catalyst carrier formed of an oxide ceramic such ascordierite.

However, the catalyst carrier formed of the oxide ceramic easilydegrades by corrosion of the alkali metal or alkali earth metal(hereinafter referred to as “the alkali metal or the like”) which isactivated at a high temperature by the exhaust gas, especially lithium,sodium, potassium, calcium or the like, and there has been a problemthat cracks are generated in the catalyst carrier, when the degradationprogresses. Since the alkali metal or the like reacts with the catalystcarrier and is consumed, there has also been a problem that a catalystperformance drops with time.

To solve the problems, a method has been proposed in which the surfaceof the catalyst carrier is coated with a certain coating layer, and theNO_(x) trap catalyst is carried on the coating layer (e.g., JapanesePatent Application Laid-Open Nos. 10-137590, 2002-59009, etc.).According to these methods, diffusion of the alkali metal or the like tothe catalyst carrier, and further reaction of the alkali metal or thelike with the catalyst carrier are suppressed by the coating layer, itis possible to avoid the above-described problems.

Additionally, in recent years, a nonoxide ceramic such as siliconcarbide has been noted as a material constituting the catalyst carrier.The nonoxide ceramic is superior in heat resistance or chemicaldurability, and does not easily react with the alkali metal or the likewhich is the NO_(x) trap catalyst even at the high temperature by theexhaust gas. Therefore, unlike the oxide ceramic, the problems do notoccur that the cracks are generated in the catalyst carrier and thecatalyst activity drops.

However, in actual, when the alkali metal or the like is carried by thecatalyst carrier formed of the nonoxide ceramic, any crack is notgenerated in the catalyst carrier, but there has been a problem that thecatalyst activity drops more then expected with use time.

DISCLOSURE OF THE INVENTION

The present invention has been developed in view of the above-describedproblems of the conventional technique, and an object thereof is toprovide a catalyst body which produces an advantageous effect that adrop of catalyst activity is effectively prevented as compared with aconventional catalyst in a case where an alkali metal or the like whichis an NO_(x) trap catalyst is carried on a catalyst carrier formed of anonoxide ceramic.

As a result of intensive researches to solve the above-describedproblems, the present inventor has found that the above-describedproblems can be solved, when disposing an oxide film protective layerformed of a material which does not form low-melting glass with analkali metal or the like between an oxide film unavoidably formed insome of the surfaces of aggregate particles and a catalyst layer in sucha manner as to coat at least a part of the oxide film in a porouscarrier containing a main component of a nonoxide ceramic or the like,and the present invention has been completed. That is, the presentinvention provides the following catalyst.

[1] A catalyst body comprising: a porous carrier in which a large numberof aggregate particles containing a main component of a nonoxide ceramicand/or a metal are bonded to one another while a large number of poresare disposed; and a catalyst layer carried on the porous carrier andcontaining a compound of an alkali metal and/or an alkali earth metal,wherein the porous carrier has an oxide film unavoidably formed on apart of the surface of the aggregate particles, and an oxide filmprotective layer formed of a material which does not form low-meltingglass with the alkali metal and/or alkali earth metal is furtherdisposed between the oxide film and the catalyst layer in such a manneras to coat at least a part of the oxide film.

[2] The catalyst body described in the above [1], wherein the porouscarrier contains a main component of the nonoxide ceramic containing asilicon (Si) element and/or metal silicon.

[3] The catalyst body described in the above [1] or [2], wherein theporous carrier contains a main component of at least one selected fromthe group consisting of silicon carbide (SiC), metal silicon bondedsilicon carbide (Si—SiC), and silicon nitride (Si₃N₄).

[4] The catalyst body described in any of the above [1] to [3], whereinthe oxide film contains a main component of silica (SiO₂).

[5] The catalyst body described in any of the above [1] to [4], whereinthe material which does not form the low-melting glass with the alkalimetal and/or alkali earth metal is a compound of at least one elementselected from elements belonging to the group A:

-   -   the group A: scandium (Sc), titanium (Ti), vanadium (V),        chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel        (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge),        yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), tin        (Sn), and antimony (Sb).

[6] The catalyst body described in the above [5], wherein the materialwhich does not form the low-melting glass with the alkali metal and/oralkali earth metal is a compound of at least one element selected fromthe group consisting of zirconium (Zr) and titanium (Ti) among theelements belonging to the group A.

[7] The catalyst body described in any of the above [4] to [6], whereinan oxide of the alkali metal and/or alkali earth metal, the materialwhich does not form the low-melting glass with the alkali metal and/oralkali earth metal, and silica (SiO₂) have an eutectic point at 800° C.or more.

[8] The catalyst body described in any of the above [1] to [7], whereinthe porous carrier has a porosity of 40 to 90%.

[9] The catalyst body described in any of the above [1] to [8], whereinthe catalyst layer contains a compound of at least one noble metalelement selected from the group consisting of platinum (Pt), palladium(Pd), and rhodium (Rh) in addition to the compound of the alkali metaland/or alkali earth metal.

[10] The catalyst body described in any of the above [1] to [9], whereinthe porous carrier has a honeycomb form having a plurality of cellswhich are partitioned by partition walls and which constitute channelsof a fluid.

[11] The catalyst body described in the above [10], wherein the porouscarrier further comprises plugging portions which alternately pluginlet-side and outlet-side end faces of the plurality of cells.

[12] The catalyst body described in the above [10] or [11], wherein theporous carrier comprises a plurality of honeycomb segments, and theplurality of honeycomb segments are integrally bonded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of a honeycomb-shapedporous carrier; and

FIG. 2 is a schematic diagram showing an example of the honeycomb-shapedporous carrier comprising plugging portions.

BEST MODE FOR CARRYING OUT THE INVENTION

In developing a catalyst body of the present invention, the presentinventor has first studied a reason why a catalyst activity of acatalyst carrier formed of a nonoxide ceramic drops more than expectedwith respect to use time, when the carrier carries an alkali metal orthe like, although the carrier does not originally easily react with thealkali metal or the like which is an NO_(x) trap catalyst. As a result,the inventor has found a phenomenon in which the catalyst carrier formedof the nonoxide ceramic itself does not easily react with the alkalimetal or the like, but oxide films are unavoidably formed on some of thesurfaces of aggregate particles constituting the catalyst carrier, andthe oxide film reacts with the alkali metal or the like to formlow-melting glass. Since the alkali metal or the like is taken into thelow-melting glass by the phenomenon, the catalyst activity rapidlydrops.

Therefore, in the present invention, in the porous carrier containingthe main component of the nonoxide ceramic or the like, an oxide filmprotective layer formed of a material which does not form thelow-melting glass with the alkali metal or the like is disposed betweenthe oxide film unavoidably formed on a part of the surface of theaggregate particles and a catalyst layer in such a manner as to coat atleast a part of the oxide film. By this constitution, the reaction ofthe oxide film with the alkali metal or the like can be suppressed.Therefore, even in a case where the alkali metal or the like is carriedas an NO_(x) trap catalyst on the catalyst carrier formed of thenonoxide ceramic, it is possible to effectively prevent the drop of thecatalyst activity.

A best mode for carrying out the catalyst body of the present inventionwill be specifically described hereinafter. It is to be noted that the“main component” mentioned in the present description means that thecomponent occupies 50% or more by mass with respect to a total mass ofall constituting components.

According to the present invention, there is provided a catalyst bodycomprising: a porous carrier in which a large number of aggregateparticles containing a main component of a nonoxide ceramic and/or ametal are bonded to one another while the particles have a large numberof pores; and a catalyst layer carried on the porous carrier andcontaining a compound of an alkali metal and/or an alkali earth metal,wherein the porous carrier has an oxide film unavoidably formed on apart of the surface of the aggregate particles, and an oxide filmprotective layer formed of a material which does not form low-meltingglass with the alkali metal and/or alkali earth metal is furtherdisposed between the oxide film and the catalyst layer in such a manneras to coat at least a part of the oxide film.

(1) Porous Carrier

The “porous carrier” mentioned in the present invention is a carrier forcarrying a catalyst layer, and comprises a porous member in which alarge number of aggregate particles are bonded to one another whilehaving a large number of pores.

The “aggregate particles” constituting the “porous carrier” mentioned inthe present invention are objects containing the main component of thenonoxide ceramic and/or metal.

Moreover, an object of the present invention is to suppress the reactionof the oxide film unavoidably formed on at least a part of the surfaceof the aggregate particles with the alkali metal or the like carried asthe NO_(x) trap catalyst. The aggregate particles need to haveproperties that the oxide films are unavoidably formed.

Therefore, the “porous carrier” in the present invention preferablycomprises a nonoxide ceramic containing silicon element and/or metalsilicon which is the main component. More specifically, the carrierpreferably comprises a main component of at least one material selectedfrom silicon carbide, metal silicon combined silicon carbide, andsilicon nitride. The constitution of the present invention is especiallyeffective in a case where the main component of the above-describedoxide film is silica having high reactivity with the alkali metal or thelike, but the oxide film containing a main component of silica isunavoidably formed on the surface of the nonoxide ceramic containing thesilicon element or metal silicon.

The “porous carrier” in the present invention has a porosity ofpreferably 40 to 90%, further preferably 45 to 80%, especiallypreferably 50 to 70%. When the porosity is less than the range, asdescribed later, there is unfavorably a possibility that a pressure lossincreases in a case where a filter function is imparted to the catalyst.When exceeding the range, a necessary strength cannot be obtained inpractical use. The porosity can be controlled by firing temperature ormaterial blend composition. For example, when a ratio of the nonoxideceramic or the like is decreased, and a glass phase is increased, adense material having a small porosity can be prepared. Conversely, whenan organic material (graphite, starch, etc.) is added to a raw material,and burnt out during firing to form pores, a porous material having alarge porosity can be prepared. It is to be noted that the “porosity”mentioned in the present description is, needless to say, a porosity ina state before carrying the catalyst layer, and means a value measuredby Archimedes process.

Moreover, in the catalyst body of the present invention, a shape of theporous carrier is not especially limited, and forms usually for use asthe catalyst carrier may be used such as pellets, beads, rings, andfoams. A honeycomb-shaped carrier is preferable which is provided with aplurality of cells partitioned by partition walls and constituting fluidchannels, because filter characteristics (pressure loss, etc.) can bedesigned with high precision in a case where the function of the filteris imparted to the catalyst body as described later.

The “honeycomb shape” mentioned in the present description means a shapewhich is partitioned by remarkably thin partition walls 4 to form aplurality of cells 3 constituting fluid channels as in a porous carrier1 shown in FIG. 1. The whole shape of the honeycomb is not especiallylimited. For example, in addition to a cylindrical shape shown in FIG.1, examples of the shape include square pole shape, a triangle poleshape and the like.

Moreover, a honeycomb cell shape (cell shape in a section vertical to acell forming direction) is not especially limited. For example, inaddition to square cells shown in FIG. 1, examples of the cell include ahexagonal cell, a triangular cell and the like. In a circular cell, aquadrangular cell, or a polygonal cell, a thick catalyst is preventedfrom being attached to a corner portion in a cell section, and thicknessof the catalyst layer can be uniform. Considering from a cell density,numerical aperture and the like, the hexagonal cell is preferable.

A honeycomb cell density is not especially limited, and is preferably ina range of 6 to 1500 cells/square inch (0.9 to 233 cells/cm²) for use asthe catalyst carrier as in the present invention. The thickness of thepartition wall is preferably in a range of 20 to 2000 μm.

Furthermore, in the catalyst body of the present invention, when theporous carrier has the above-described honeycomb shape, the catalystbody preferably further comprises plugging portions for alternatelyplugging inlet-side and outlet-side end faces of a plurality of cells.By this memory, the function of the filter can be imparted to thecatalyst body (catalyst carrying filter).

For example, as shown in FIG. 2, according to a porous carrier 21further comprising plugging portions 22 which alternately plug aninlet-side end face B and an outlet-side end face C of a plurality ofcells 23, when a gas G₁ to be treated is introduced into the cells 23from the inlet-side end face B, dust or particulates are captured inpartition walls 24. On the other hand, a treated gas G₂ which has passedthrough the porous partition wall 24 to flow into the adjacent cell 23is discharged from the outlet-side end face C, and therefore the treatedgas G₂ can be obtained from which the dust or particulates in the gas G₁to be treated have been separated.

Furthermore, in the catalyst body of the present invention, when theporous carrier has the above-described honeycomb shape, the carrierpreferably comprises a plurality of honeycomb segments, and theplurality of honeycomb segments are integrally bonded (bonded member).As the ceramic constituting the porous carrier in the catalyst body orcatalyst carrying filter, cordierite which is an oxide ceramic isrepresentative. Since the nonoxide ceramic constituting the porouscarrier in the catalyst body of the present invention has a largecoefficient of thermal expansion as compared with cordierite, a heatstress by a temperature distribution increases. Therefore, by thestructure of the above-described bonded member, the thermal stress isreleased, cracks by the thermal stress can be prevented, and a thermalshock resistance of the catalyst body can be improved.

When the catalyst body comprises a plurality of honeycomb segments, thesize of each segment is not limited. However, when each segment isexcessively large, an effect of improving the thermal shock resistanceis reduced. On the other hand, when the segment is excessively small,integration of the respective segments by manufacturing or bonding isunfavorably complicated. Considering from this respect, as to a size ofeach segment, a sectional area (section vertical to a cell formingdirection) is preferably 900 to 10000 mm², further preferably 900 to5000 mm², especially preferably 900 to 3600 mm², and 70% or more byvolume of the catalyst body preferably comprises the segment having thissize.

As to the “porous carrier” in the present invention, for example,aggregate particle materials comprising the nonoxide ceramic and/ormetal, and water are, if desired, mixed and kneaded with an organicbinder (hydroxypropoxyl methyl cellulose, methyl cellulose, etc.), poreformer (graphite, starch, synthetic resin, etc.), surfactant (ethyleneglycol, fatty acid soap, etc.) and the like to form clay. The clay isformed into a desired shape, and dried to thereby obtain a formedarticle, the formed article is calcined to form a calcined article, andthereafter the calcined article can be fired to obtain the carrier.

It is to be noted that as a method of forming the porous carrier intothe honeycomb shape, a method or the like is preferably usable in whichthe clay prepared as described above is extruded using a ferrule havinga desired cell shape, partition wall thickness, and cell density. As amethod in which the plugging portions are disposed in such a manner asto alternately plug the inlet-side and outlet-side end faces of aplurality of cells, there is a method or the like in which thehoneycomb-shaped porous carrier is formed by extrusion, and dried, andthereafter cell openings are filled with clay having the samecomposition as that of the clay for forming.

(2) Oxide Film Protective Layer

The “oxide film protective layer” mentioned in the present descriptionis constituted of the material which does not form the low-melting glasswith the alkali metal and/or alkali earth metal, and is formed in such amanner as to coat at least a part of the above-described oxide film. Ina portion in which this oxide film protective layer is disposed betweena part of the oxide film and the catalyst layer, the above-describedoxide film can be securely isolated from the catalyst layer, and theoxide film can be inhibited from being reacted with the alkali metal orthe like carried as the NO_(x) trap catalyst. Therefore, even when thealkali metal or the like is carried by the catalyst carrier formed ofthe nonoxide ceramic or the like, the catalyst activity can beeffectively prevented from being degraded.

It is to be noted that in the present invention, since the porouscarrier comprises the nonoxide ceramic or the like, the constitutingmaterial itself of the porous carrier does not react with the alkalimetal or the like. Therefore, a constitution is not essential in whichthe whole surface of the porous carrier is coated with the oxide filmprotective layer, and a characteristic lies in that at least a part ofthe oxide film is sufficiently coated. On the other hand, as to theoxide ceramic (cordierite, etc.) which has heretofore been used as theconstituting material of the catalyst carrier, the constituting materialof the catalyst carrier itself reacts with the alkali metal or the like,and therefore the constitution is required in which the whole surface ofthe catalyst carrier is coated with a certain coating layer.

Specifically, examples of the “material which does not form thelow-melting glass with the alkali metal and/or alkali earth metal”include a compound of at least one element selected from the followingelements belonging to the group A. Since it is comparatively easy to useparticulates such as oxide colloidal particles (zirconia (ZrO₂) sol,titania (TiO₂) sol, etc.) in forming the oxide film protective layer, acompound of at least one element selected from zirconium and titanium ispreferable among the following elements belonging to the group A (e.g.,zirconia, titania, etc.).

The group A: scandium, titanium, vanadium, chromium, manganese, iron,cobalt, nickel, copper, zinc, gallium, germanium, yttrium, zirconium,niobium, molybdenum, tin, and antimony.

More specifically, the “low-melting glass is not formed with the alkalimetal and/or alkali earth metal” means that the oxide of the alkalimetal and/or alkali earth metal, the “material which does not form thelow-melting glass with the alkali metal and/or alkali earth metal”, andsilica have an eutectic point at 800° C. or more. The eutectic point isset to 800° C. or more in consideration of a temperature condition in anenvironment (automotive exhaust gas system) in which the catalyst bodyis actually disposed. It is assumed that the oxide film protective layerdoes not substantially react with the alkali metal or the like, when theeutectic point is 800° C. or more. Therefore, it is possible to suppressthe reaction of the oxide film with the alkali metal or the like, andthe drop of the catalyst activity can be effectively prevented.

A method of forming the oxide film protective layer is not especiallylimited. For example, there is a method or the like in which afterwash-coating the porous carrier with a coating solution containing the“material that does not form the low-melting glass with the alkali metaland/or alkali earth metal”, the carrier is thermally treated at hightemperature and baked.

(3) Catalyst Layer

The “catalyst layer” mentioned in the present invention is a layercontaining a compound of an alkali metal and/or alkali earth metal whichis an NO_(x) trap catalyst, and carried by the porous carrier.

In the catalyst body of the present invention, a type of the alkalimetal or alkali earth metal carried as the NO_(x) trap catalyst is notespecially limited, examples of the alkali metal include lithium,sodium, potassium, cesium and the like, and examples of the alkali earthmetal include calcium, barium, strontium and the like. Above all,potassium is especially preferably usable, because it is superior inNO_(x) trap ability in a high-temperature range.

Moreover, the catalyst layer mentioned in the present invention maycontain a compound of at least one noble metal element selected fromplatinum, palladium, and rhodium in addition to the compound of theabove-described alkali metal and/or alkali earth metal. By these noblemetals, nitrogen monoxide (NO) in the exhaust gas is reacted with oxygen(O₂) to generate nitrogen dioxide (NO₂) before the alkali metal or thelike traps nitrogen oxide. When once trapped nitrogen oxide isdischarged, combustible components in the exhaust gas is reacted withnitrogen oxide to detoxify nitrogen oxide, and a capability of purifyingnitrogen oxide is preferably enhanced. Since catalyst components such asalkali metals and noble metals are carried in highly scattered states,the components are preferably once carried by heat-resistant inorganicoxide having a large specific surface area, like alumina, and thereaftercarried by the porous carrier.

As to the catalyst body of the present invention, another purifyingmaterial applicable to an exhaust gas system may be carried togetherwith the NO_(x) trap catalyst, the purifying material including acatalyst component other than the NO_(x) trap catalyst represented by atertiary catalyst, auxiliary catalyst represented by oxide of cerium(Ce) and/or zirconium, hydro carbon (HC) adsorbing material or the like.In this case, the NO_(x) trap catalyst and these purifying materials maybe carried in a mixed state, but the respective components are morepreferably stacked and carried in such a manner as to form independentlayers. Furthermore, the NO_(x) trap catalyst and these purifyingmaterials are carried by separate carriers, and these carriers arepreferably appropriately combined for use in the exhaust gas system.

A method of forming the catalyst layer is not especially limited, but,for example, there is a method or the like in which the porous carrierincluding the oxide film protective layer formed thereon is wash-coatedwith a catalyst solution containing catalyst components, thereafterthermally treated at high temperature, and baked.

EXAMPLES

The present invention will be described hereinafter more specifically inaccordance with examples, but the present invention is not limited tothese examples. It is to be noted that as to an average particlediameter of an aggregate particle material in the following examples andcomparative examples, a value of a 50% particle diameter was used. Thevalue was measured by an X-ray transmission type grain-size distributionmeasurement apparatus (e.g., Sedigraph 5000-02 type, etc. manufacturedby Shimazu Corp.) in which Stokes liquid-phase sedimentation process isused as a measuring principle, and detection is performed by an X-raytransmission process.

[Manufacturing of Porous Carrier]

(Carrier 1)

As aggregate particles, a total of 100 parts by mass were preparedincluding: 80 parts by mass of silicon carbide having an averageparticle diameter of 50 μm; and 20 parts by mass of metal silicon powderhaving an average particle diameter of 5 μm. Moreover, 10 parts by massof hydroxypropyl methyl cellulose which was an organic binder, 10 partsby mass of starch which was a pore former, and an appropriate amount ofwater were added to 100 parts by mass of aggregate particles, andmixed/kneaded by a vacuum kneader to prepare clay.

By an extruding method using a ferrule having a cell shape, partitionwall thickness, and cell density described later, the above-describedclay was formed into a honeycomb shape, and thereafter dried by a dryingmethod by combination of hot-air drying and microwave drying to obtain ahoneycomb-shaped formed article. As the whole shape of the obtainedformed article, an end face (cell opening face) had a 35 mm×35 mm squareshape, a length was 152 mm, each cell had a 1.2 mm×1.2 mm square shape,each partition wall had a thickness of 310 μm, a cell density was 46.5cells/cm² (300 cells/square inch), and a total cell number was 576cells.

The formed article was calcined (degreased) at about 400° C. for fivehours in the atmosphere to thereby obtain a calcined article, and thiscalcined article was fired in an argon atmosphere at about 1450° C. fortwo hours to obtain a porous carrier (metal silicon bonded siliconcarbide). In the porous carrier, a porosity measured by Archimedesprocess was 52%, and an average pore diameter measured by a mercurypress-in process was 20 μm. This “porous carrier” is referred to as“Carrier 1”.

(Carrier 2)

As aggregate particles, 100 parts by mass of metal silicon powder wereprepared having an average particle diameter of 20 μm. Moreover, 10parts by mass of hydroxypropyl methyl cellulose which was an organicbinder, 10 parts by mass of starch which was a pore former, and anappropriate amount of water were added to 100 parts by mass of aggregateparticles, and mixed/kneaded by a vacuum kneader to prepare clay.

By an extruding method using a ferrule having a cell shape, partitionwall thickness, and cell density described later, the above-describedclay was formed into a honeycomb shape, and thereafter dried by a dryingmethod by combination of hot-air drying and microwave drying to obtain ahoneycomb-shaped formed article. As the whole shape of the obtainedformed article, an end face (cell opening face) had a 35 mm×35 mm squareshape, a length was 152 mm, each cell had a 1.2 mm×1.2 mm square shape,each partition wall had a thickness of 310 μm, a cell density was 46.5cells/cm² (300 cells/square inch), and a total cell number was 576cells.

The formed article was calcined (degreased) at about 400° C. for fivehours in the atmosphere to thereby obtain a calcined article, and thiscalcined article was fired in a nitrogen atmosphere at about 1450° C.for two hours to obtain a porous carrier (silicon nitride). In theporous carrier, a porosity measured by Archimedes process was 52%, andan average pore diameter measured by a mercury porosimeter was 10 μm.This “porous carrier” is referred to as “Carrier 2”.

[Forming of Oxide Film Protective Layer]

Carriers 1 and 2 described above were wash-coated with titania sol of acommercially available nitric acid solution or zirconia sol of thenitric acid solution, and accordingly an oxide film protective layer wasformed in such a manner as to coat at least a part of an oxide filmformed on the surfaces of the aggregate particles constituting theabove-described carriers. A coating amount was defined in terms of massper unit volume of the porous carrier, and any amount of 5 g/L, 25 g/L,50 g/L was used. When the coating amount did not reach a predeterminedvalue by one wash-coating time, the wash-coating was repeated until thepredetermined value was reached. Thereafter, thermal treatment wasperformed on a condition at 700° C. for 1 hour to perform baking.

[Preparation of Catalyst Solution and Formation of Catalyst Body]

A commercially available γ-alumina (γ-Al₂O₃) powder (specific surfacearea: 200 m²/g) was immersed in a solution obtained by mixing an aqueoussolution of diamino platinum nitrite ((NH₃)₂Pt(NO₂)₂) and an aqueoussolution of potassium nitrate (KNO₃), and stirred in a pot mill for twohours. Thereafter, a water content was evaporated, and the material wasdried/solidified, soft-crushed, and fired in an electric furnace at 600°C. for 3 hours. A commercially available alumina (Al₂O₃) sol and watercontent were added to thus obtained (platinum+potassium) containingγ-alumina powder ((Pt+K)-predoped γ-Al₂O₃), and again wet-crushed in thepot mill to prepare a catalyst solution (slurry for wash-coating).

A carrying amount of potassium was adjusted into 20 g/L per porouscarrier volume. An amount relation between γ-alumina, platinum, andpotassium was adjusted in such a manner that a potassium carrying amountwas 20 g/L (per porous carrier volume) and a platinum amount was 30g/cft (1.06 g/L) (per porous carrier volume, mass of a platinum elementbase) in a stage in which the porous carrier constituting the catalystcarrier was wash-coated with a catalyst solution, and finally subjectedto a heat treatment. As to an added amount of an alumina sol, an amountof a solid content was set to 5 mass % of total alumina in terms ofalumina, and the water content was appropriately added in such a mannerthat the catalyst solution indicated such a viscosity that facilitatesthe wash-coating.

The porous carrier constituting the catalyst carrier was immersed in theobtained catalyst solution, an excess solution in the cell was blownoff, and the cells were dried. The obtained potassium carrier wasthermally treated in the electric furnace at 600° C. for 1 hour toprepare a catalyst body. It is to be noted that any oxide filmprotective layer was not formed, and a catalyst layer was formed onCarrier 1 to obtain Comparative Example 1, whereas any oxide filmprotective layer was not formed, and a catalyst layer was formed onCarrier 2 to obtain Comparative Example 2.

(Evaluation of Potassium Diffusion Suppression Degree)

With regard to each catalyst body, while 10% (volume %) of a watercontent coexisted, an acceleration durability test was performed in sucha manner as to retain the catalyst body at 750° C. for 30 hours.Before/after the testing, a degree of scattering of potassium (potassiumdiffusion suppression degree) was evaluated by a potassium concentrationdistribution diagram measured by an energy scattering type spectrometer.Results are shown in Table 1. It is to be noted that in the evaluation,the diffusion degree of potassium before/after the accelerationdurability test was used as a standard. When potassium hardly diffused,and the degree was substantially equal to that before the test, thedegree was evaluated as A. When potassium slightly diffused, the degreewas evaluated as B. When most of potassium diffused, the degree wasevaluated as C. When potassium hardly remained in its original position,the degree was evaluated as D.

TABLE 1 Oxide film Evaluation protective layer Potassium Coatingdiffusion Porous Substance amount suppression carrier name (g/L) degreeExample 1 Carrier 1 Titania 5 B Example 2 Carrier 1 Titania 25 A Example3 Carrier 1 Titania 50 A Example 4 Carrier 1 Zirconia 25 A Example 5Carrier 1 Zirconia 50 A Example 6 Carrier 2 Titania 25 A Example 7Carrier 2 Zirconia 25 A Comparative Carrier 1 None — D Example 1Comparative Carrier 2 None — D Example 2(Result)

As apparent from the results shown in Table 1, it has been found thatthe diffusion of potassium can be effectively suppressed in the catalystbodies of Examples 1 to 7. That is, it has been supposed that a drop ofcatalyst activity can be effectively prevented in a case where an alkalimetal or the like that is an NO_(x) trap catalyst is carried by acatalyst carrier formed of a nonoxide ceramic and/or a metal. On theother hand, in the catalyst bodies of Comparative Examples 1 and 2, thediffusion of potassium can hardly be suppressed. It is to be noted thatalthough not shown in Table 1, similar results were indicated even in acase where the catalyst component was changed from potassium to sodiumor lithium, and evaluated.

INDUSTRIAL APPLICABILITY

As described above, in a catalyst body of the present invention, anoxide film protective layer formed of a material which does not formlow-melting glass with an alkali metal or the like is disposed betweenan oxide film unavoidably formed on a part of the surface of anaggregate particle, and a catalyst layer in such a manner as to coat atleast a part of the oxide film in a porous carrier containing a maincomponent of a nonoxide ceramic. Therefore, it is possible toeffectively prevent a drop of catalyst activity in a case where thealkali metal or the like which is an NO_(x) trap catalyst is carried bya catalyst carrier formed of the nonoxide ceramic.

1. A catalyst body comprising: a porous carrier in which a large numberof aggregate particles containing a main component of a nonoxide ceramicand/or a metal are bonded to one another while a large number of poresare disposed; and a catalyst layer carried on the porous carrier andcontaining a compound of an alkali metal and/or an alkali earth metal,wherein the porous carrier consists essentially of aggregate particlesbonded to each other, an oxide film unavoidably formed from material ofthe aggregate particles on a part of the surface of the aggregateparticles, and an oxide film protective layer formed of a material whichdoes not form low-melting glass with the alkali metal and/or alkaliearth metal used as a NO_(X) trap catalyst that is further disposed onthe oxide film so as to coat at least a part of the oxide film.
 2. Thecatalyst body according to claim 1, wherein the porous carrier containsa main component of the nonoxide ceramic containing a silicon (Si)element and/or metal silicon.
 3. The catalyst body according to claim 1,wherein the porous carrier contains a main component of at least oneselected from the group consisting of silicon carbide (SiC), metalsilicon bonded silicon carbide (Si—SiC), and silicon nitride (Si₃N₄). 4.The catalyst body according to claim 1, wherein the oxide film containsa main component of silica (SiO₂).
 5. The catalyst body according toclaim 4, wherein an oxide of the alkali metal and/or alkali earth metal,the material which does not form the low-melting glass with the alkalimetal and/or alkali earth metal, and silica (SiO₂) have an eutecticpoint at 800° C. or more.
 6. The catalyst body according to claim 1,wherein the material which does not form the low-melting glass with thealkali metal and/or alkali earth metal is a compound of at least oneelement selected from elements belonging to the group A: the group A:scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese(Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn),gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), niobium (Nb),molybdenum (Mo), tin (Sn), and antimony (Sb).
 7. The catalyst bodyaccording to claim 6, wherein the material which does not form thelow-melting glass with the alkali metal and/or alkali earth metal is acompound of at least one element selected from the group consisting ofzirconium (Zr) and titanium (Ti) among the elements belonging to thegroup A.
 8. The catalyst body according to claim 1, wherein the porouscarrier has a porosity of 40 to 90%.
 9. The catalyst body according toclaim 1, wherein the catalyst layer contains a compound of at least onenoble metal element selected from the group consisting of platinum (Pt),palladium (Pd), and rhodium (Rh) in addition to the compound of thealkali metal and/or alkali earth metal.
 10. The catalyst body accordingto claim 1, wherein the porous carrier has a honeycomb form having aplurality of cells which are partitioned by partition walls and whichconstitute channels of a fluid.
 11. The catalyst body according to claim10, wherein the porous carrier further comprises plugging portions whichalternately plug inlet-side and outlet-side end faces of the pluralityof cells.
 12. The catalyst body according to claim 10, wherein theporous carrier comprises a plurality of honeycomb segments, and theplurality of honeycomb segments are integrally bonded.
 13. The catalystbody according to claim 1, wherein the aggregate particles include amain component comprising of at least one material selected from siliconmetal, silicon carbide or a mixture thereof.